1. Field of the Invention
The present invention generally relates to image capture systems and methods and, more particularly, to generating well-defined aiming light patterns on optical code symbols prior to their being electro-optically imaged and read.
2. Description of the Related Art
Optical codes are patterns made up of image areas having different light-reflective or light-emissive properties, which are typically assembled in accordance with a priori rules. The term “barcode” is sometimes used to describe certain kinds of optical codes. The optical properties and patterns of optical codes are selected to distinguish them in appearance from the background environments in which they are used. Devices for identifying or extracting data from optical codes are sometimes referred to as “optical code readers” of which barcode scanners are one type. Optical code readers are used in both fixed or portable installations in many diverse environments such as in stores for check-out services, in manufacturing locations for work flow and inventory control, and in transport vehicles for tracking package handling. The optical code can be used as a rapid, generalized means of data entry, for example, by reading a target barcode from a printed listing of many barcodes. In some uses, the optical code reader is connected to a portable data processing device or a data collection and transmission device. Frequently, the optical code reader includes a handheld sensor which is manually directed at a target code.
Most conventional optical scanning systems are designed to read one-dimensional barcode symbols. The barcode is a pattern of variable-width rectangular bars separated by fixed or variable width spaces. The bars and spaces have different light-reflecting characteristics. One example of a one-dimensional barcode is the UPC/EAN code used to identify, for example, product inventory. An example of a two-dimensional or stacked barcode is the PDF417 barcode. A description of PDF417 barcode and techniques for decoding it is disclosed in U.S. Pat. No. 5,635,697. Another conventional optical code is known as “MaxiCode”. It consists of a central finder pattern or bull's eye center and a grid of hexagons surrounding the central finder. It should be noted that the aspects of the inventions disclosed in this patent application are applicable to optical code readers, in general, without regard to the particular type of optical codes which they are adapted to read. The invention described is also applicable to some associated image recognition or analysis.
Most conventional scanning systems generate one or more beams of laser light which reflects off a barcode symbol and back to the system. The system obtains a continuous analog waveform corresponding to the light reflected by the code along one or more scan lines of the system. The system then decodes the waveform to extract information from the barcode. A system of this general type is disclosed, for example, in U.S. Pat. No. 4,251,798. A beam scanning system for detecting and decoding one-and two-dimensional barcodes is disclosed in U.S. Pat. No. 5,561,283.
Many scanners in use today employ a moving laser beam. Some such systems are deployed in hand-held units which may be manually pointed at a target. Often an individual scanner is a component of a much larger system including other scanners, computers, cabling, data terminals, etc.
Barcodes can also be read by employing imaging devices, also deployed in hand-held units. For example, an image sensor may be employed which has a one- or two-dimensional array of cells or pixel sensors which correspond to image elements or pixels in a field of view of the device. Such an image sensor may be a two-dimensional or area charge coupled device (CCD) and associated circuits for producing electronic signals corresponding to a two-dimensional array of pixel information for a field of view.
It is therefore known to use a CCD for capturing a monochrome image of a barcode symbol to be read as, for example, disclosed in U.S. Pat. No. 5,703,349. It is also known to use a CCD with multiple buried channels for capturing a full color image of a target as, for example, disclosed in U.S. Pat. No. 4,613,895.
Although generally satisfactory for its intended purpose, the use of an imaging device is frustrated because an operator cannot tell whether the device, or the hand-held unit in which it is mounted, is aimed directly at the barcode. Contrary to moving beam scanners in which the operator can see the beam on the barcode, the imaging device is a passive unit and provides no visual feedback to the operator.
To alleviate such problems, the prior art has proposed in U.S. Pat. No. 6,060,722 an aiming pattern generator for an imaging reader. This known generator utilizes a diffractive element, a holographic element, or a Fresnel element, which generate a light interference pattern. However, such interferometric pattern generators have several limitations, including diffraction power losses and non-uniform or dotted pattern lines.
Also, the maximum angular spread of the interferometric pattern over the field of view is limited by the minimum dimension of each structure of the interferometric element. By way of example, it is known to provide a diffraction grating with periodic structures each having a minimum dimension Tmin. To obtain a maximum angular spread αmax, it is known that Tmin=λ/sin (αmax), where λ is the wavelength of the light. In a typical application, αmax=20 degrees, and λ=0.65 microns, and therefore, Tmin=1.9 microns. The periodic structures need to be dimensioned to be about half of Tmin or 0.95 microns. To achieve these dimensions, photoetching is typically required, which however is costly and difficult to perform. For even greater angular spreads, the situation is aggravated and the dimension Tmin reaches practical fabrication limits.
It is also known to use non-interferometric optical elements to project an aiming line. For example, U.S. Pat. No. 6,069,748 discloses the use of a toroidal lens to project a single aiming line to guide a cutting tool. Such technology, however, is not suitable for electro-optical readers, especially imaging readers operative for reading two-dimensional barcodes, and produces the aiming line with non-uniform light intensity distribution.